U.S. Pat. No. 4,188,734 to Rich discloses an educational toy having a board with a plurality of holes on a front surface and a plurality of holes in a back surface. The board includes a first set of indicia associated with the holes in the front surface and a second set of indicia associated with the holes in the back surface. The toy includes a plurality of pegs sized to be received within the holes, wherein each peg includes permanent indicia associating the peg with a given hole in the front surface and the back surface of the board.
U.S. Pat. No. 4,526,550 to Lurito discloses a combination system including a plurality of receivers and plugs having cooperable coding features which are coded according to unique rules such that a desired form of cooperation between the plug and the corresponding receiver is obtained only when the specific plug matches the code.
U.S. Pat. No. 3,374,558 to Smith discloses an educational aperture board having openings therein, a plurality of opaque overlay cards wherein each card includes a set of openings alignable with the openings in the aperture board, and a plurality of pegs sized to be received through the overlay and into the aperture board.